13 3 Digestion Digestive Systems BIO361T Fall 2014 1 Gut reactor theory can be used to predict optimal retention time for food but it does not take into account physiological variables that may vary What are three factors that could change opt Effects of digestive enzymes active transport proteins in the membrane Volume of gut varies with contraction and as fluid is reabsorbed in the colon large intestine Thorough mixing does not occur unstirred layers form Functional length of gut also varies with longitudinal muscle contraction Multiple contributors to most diets each of which may have different opt and may take longer to pass Toxic materials will be actively pushed through 2 About 25 of teleost fish have lost their stomachs over evolutionary time as have monotremes echidna platypus Given the functions of the stomach propose two ultimate explanations for the evolutionary loss of the stomach Change in pepsinogen needs due to dietary differences Lack of need for acidic pH Many of these animals live in alkaline environments and eat shells corals and mud These might neutralize the stomach acid making it irrelevant Acid secretion parietal cells could accrue mutations http phenomena nationalgeographic com 2013 12 03 how the platypus and a quarter offisheslost their stomachs 3 Why do you think the cecum of a grouse is twice as long as the cecum of a similarly sized partridge Recall that the cecum often contains enterosymbiotic bacteria It is enlarged in animals that have diets that are difficult to digest The grouse eats plants while the partridge eats seeds 4 The electrochemical gradient for Na is commonly used to drive transport of other solutes during digestion such as the monosaccharides and amino acids liberated from macromolecules in food a Where do monosaccharides come from and what type of enzyme liberates them From polysaccharides amylase b Where do amino acids come from and what type of enzyme liberates them From proteins protease c This process is shown in the vertebrate midgut below Is the apical transport of fructose passive or active Passive d Why does the apical transport of glucose require a cotransporter symporter but fructose does not Concentration of glucose is higher inside cell than out opposite is true of fructose
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